Geography

Basin overview

Snake River's drainage basin includes a diversity of landscapes. Its upper reaches lie in the Rocky Mountains. In central Idaho the river flows through the broad Snake River Plain. Along the Idaho-Oregon border the river flows through Hells Canyon, part of a larger physiographic region called the Columbia River Plateau, through which the Snake River flows through Washington to its confluence with the Columbia River. Parts of the river's basin lie within the Basin and Range province, though it is itself a physiographic section of the Columbia Plateau province, which in turn is part of the larger Intermontane Plateaus physiographic division.

The Snake is the largest tributary of the Columbia River, with a mean discharge of , or according to the USGS, the 12th largest in the United States.

Geology

For much of its course the Snake River flows through the Snake River Plain, a physiographic province extending from eastern Oregon through western and central Idaho into northwest Wyoming. Much of the Snake River Plain is high desert and semi-desert at elevations averaging around . Many of the rivers in this region have cut deep and meandering canyons. West of Twin Falls, the plain is mainly covered with stream and lake sediments. During the Miocene, lava dams created Lake Idaho, which covered a large portion of the Snake River Plain between Twin Falls and Hells Canyon. This large lake expanded and contracted several times before finally receding in the early Pleistocene. In more recent geologic time, about 14,500 years ago, glacial Lake Bonneville spilled catastrophically into the Snake River Plain. The flood carved deep into the land along the Snake River, leaving deposits of gravel, sand, and boulders, as well as a scabland topography in places. Results of this flood include the falls and rapids from Twin Falls and Shoshone Falls to Crane Falls and Swan Falls, as well as the many "potholes" areas.

The Snake River Aquifer, one of the most productive aquifers in the world, underlies an area of about in the Snake River Plain. Differences in elevation and rock permeability result in many dramatic springs, some of which are artesian. The groundwater comes from the Snake River itself as well as other streams in the region. Some streams on the northern side of the Snake River Plain, such as the Lost River are completely absorbed into the ground, recharging the aquifer and emerging as springs that flow into the Snake River in the western part of the plain. The hydraulic conductivity of the basalt rocks that make up the aquifer is very high. In places water exits the Snake and Lost rivers into ground conduits at rates of nearly . Due to stream modifications and large-scale irrigation, most of the water that used to recharge the aquifer directly now does so in the form of irrigation water drainage.

Below the Palisades Reservoir, the Snake River flows northwest through Swan Valley to its confluence with Henrys Fork near Rigby. The region around the confluence is a large inland delta. Above the juncture, the Snake River is locally called the South Fork of the Snake River, since Henrys Fork is sometimes called the North Fork of the Snake River.

Early inhabitants

People have been living along the Snake River for at least 11,000 years. Daniel S. Meatte divides the prehistory of the western Snake River Basin into three main phases or "adaptive systems". The first he calls "Broad Spectrum Foraging", dating from 11,500 to 4,200 years before present. During this period people drew upon a wide variety of food resources. The second period, "Semisedentary Foraging", dates from 4,200 250 years before present and is distinctive for an increased reliance upon fish, especially salmon, as well as food preservation and storage. The third phase, from 250 to 100 years before present, he calls "Equestrian Foragers". It is characterized by large horse-mounted tribes that spent long amounts of time away from their local foraging range hunting bison. In the eastern Snake River Plain there is some evidence of Clovis, Folsom, and Plano cultures dating back over 10,000 years ago. By the protohistoric and historic era, the eastern Snake River Plain was dominated by Shoshone and other "Plateau" culture tribes.

Early fur traders and explorers noted regional trading centers, and archaeological evidence has shown some to be of considerable antiquity. One such trading center in the Weiser area existed as early as 4,500 years ago. The Fremont culture may have contributed to the historic Shoshones, but it is not well understood. Another poorly understood early cultural hearth is called the Midvale Complex. The introduction of the horse to the Snake River Plain around 1700 helped in establishing the Shoshone and Northern Paiute cultures.

On the Snake River in southeastern Washington there are several ancient sites. One of the oldest and most well-known is called the Marmes Rockshelter, which was used from over 11,000 years ago to relatively recent times. The Marmes Rockshelter was flooded in 1968 by Lake Herbert G. West, the Lower Monumental Dam's reservoir.

Exploration

The Lewis and Clark Expedition of 1804-1806 was the first major U.S. exploration of the lower portion of the Snake River, and the Snake was once known as the Lewis River. Later American exploratory expeditions, which explored much of the length of the Snake River, included the Astor Expedition of 1810-1812, John C. Frémont in 1832, and Benjamin Bonneville in 1833-1834. The British North West Company and, after 1821, Hudson's Bay Company sent large trapping and trading expeditions to the upper Snake River and its tributaries. These annual expeditions began in 1817 and continued for about 30 years. They ranged widely throughout today's southern Idaho, western Wyoming, and northern Utah, in the process exploring the region. Many of the rivers and mountains have French names, reflecting the employment of French-Canadian voyageur by the British companies. The policy of the Hudson's Bay Company's was to deplete the region of fur bearing animals as quickly as possible, so as to make it impossible for American traders to operate there. This goal was largely successful. American traders could not compete in the Snake River country. In contrast, the Hudson's Bay Company found the Snake River expeditions very profitable.
By the middle 19th century, the Oregon Trail had been established, generally following much of the Snake River.

River modifications

Dams

Many dams have been built on the Snake River and its tributaries, mainly for purposes of providing irrigation water and hydroelectric power and ranging in size from small diversion dams to major high dams.

Large dams include four on the lower Snake, in Washington, built and operated by the U.S. Army Corps of Engineers: Ice Harbor, Lower Monumental, Little Goose, and Lower Granite. These dams were built from 1962 to 1975 for hydroelectric power and navigation. They are equipped with locks, making the river as far as Lewiston an extension of the Columbia River's barge navigation system. The four dams were modified in the 1980s to better accommodate fish passage.

Upriver, in the Hells Canyon region, there are three large hydroelectric dams, operated by Idaho Power, a private utility company. Collectively named the Hells Canyon Project, the three dams are, in upriver order: Hells Canyon Dam, Oxbow Dam, and Brownlee Dam. Not having fish ladders, they are the first total barrier to upriver fish migration.

In southwestern Idaho there are several large dams. Swan Falls Dam, built in 1901, was the first hydroelectric dam on the Snake as well as the first total barrier to upriver fish migration. It was rebuilt in the 1990s by Idaho Power. Upriver from Swan Falls is another hydroelectric dam operated by Idaho Power, the C. J. Strike Dam, built in 1952. This dam also serves irrigation purposes. Continuing upriver, Idaho Power operates a set of three hydroelectric dam projects collectively called the Mid-Snake Projects, all built in the 1940s and 1950s. They are: Bliss Dam, Lower Salmon Falls Dam, and the two dams of the Upper Salmon Falls Project, Upper Salmon Falls Dam A and Upper Salmon Falls Dam B.

Near the city of Twin Falls two waterfalls have been modified for hydropower, Shoshone Falls and Twin Falls. Collectively called the Shoshone Falls Project, they are old and relatively small dams, currently operated by Idaho Power. Above Twin Falls is Milner Dam, built in 1905 for irrigation and rebuilt in 1992 with hydroelectric production added. The dam and irrigation works are owned by Milner Dam, Inc, while the powerplant is owned by Idaho Power.

The city of Idaho Falls operates the remaining large dam on the Snake River, Gem State Dam, along with several smaller associated dams, for hydroelectric and irrigation purposes.

There are many other dams on the tributaries of the Snake River, built mainly for irrigation. They are mainly operated by the Bureau of Reclamation or local government and private owners.

While the many dams in the Snake River basin have transformed the region's economy, they have also had an adverse environmental effect on wildlife, most notably on wild salmon migrations. Since the 1990s, some conservation organizations and fishermen are seeking to restore the lower Snake River and Snake River salmon and steelhead by removing four federally-owned dams on the lower Snake River.

Navigation

In the 1960s and 1970s the U.S. Army Corps of Engineers built four dams and locks on the lower Snake River to facilitate shipping. The lower Columbia River has likewise been dammed for navigation. Thus a deep shipping channel through locks and slackwater reservoirs for heavy barges exists from the Pacific Ocean to Lewiston, Idaho. Most barge traffic originating on the Snake River goes to deep-water ports on the lower Columbia River, such as Portland. Grain, mostly wheat, is the main product shipped from the Snake, and nearly all of it is exported internationally from the lower Columbia River ports.

The shipping channel is authorized to be at least deep and wide. Where river depths were less than , the shipping channel has been dredged in most places. Dredging and redredging work is ongoing and actual depths vary over time.

With a channel about deeper than the Mississippi River System, the Columbia and Snake rivers can float barges twice as heavy.

Agricultural products from Idaho and eastern Washington are among the main goods transported by barge on the Snake and Columbia rivers. Grain, mainly wheat, accounts for more than 85% of the cargo barged on the lower Snake River. In 1998, over of grain were barged on the Snake. Before the completion of the lower Snake dams, grain from the region was transported by truck or rail to Columbia River ports around the Tri-Cities. Other products barged on the lower Snake River include peas, lentils, forest products, and petroleum.

Among the negative consequences of the lower Snake River's navigational slackwater reservoirs are the flooding of historic and archaeological sites, the stilling of once famous rapids, the slowing of currents and an associated rising of water temperature, and a general decline in the ability of wild fish to migrate up and down the river.